Anti-surge compressor capacity control

ABSTRACT

APPARATUS FOR CONTROLLING THE OPERATION OF A CENTRIFUGAL COMPRESSOR BY DERIVING A SIGNAL FROM THE HIGH SIDE OF THE SYSTEM AND USING SUCH SIGNAL TO RESET THE CONTROL POINT WHICH LIMITS CLOSING OF THE PRV AND INITIATES BYPASSING DISCHARGE GAS. AT LOWER CAPACITY, THE SYSTEM OPERATES CLOSE TO THE SURGE LINE ASSURING MAXIMUM ECONOMY.

Jan. 19, 1971 J. E. FLECKENSTEIN ET AL 3,555,844

I ANTI-SURGE COMPRESSOR CAPACITY CONTROL Filed Jan. 2, 1969 HEAD ,- GAS mow i r2 ,To HOT eA s VALVE TO pszv PRV 74 COEIROL A"? c my 28% 29 SIGNAL Q wssasa F1 1 Ms? as 44 AUTO-MANUAL S comma-J 26 Q k2? -.-CQNDEN$ER v INVENTORS E URE JUSEPf/TFZECAE/VSTE/N 50 1 azMAn ask/ F257 2 BY #52444 ,5. PAETOW ATTORNEY jam 5 :FPA/II/A/A M14254 United States Patent O ANTI-SURGE COMPRESSOR CAPACITY CONTROL Joseph E. Fleckenstein, Red Lion, Frank A. Kimpel and Leo J. Ormanoski, York, and Herman E. Paetow, Manchester, Pa., assignors to Borg-Warner Corporation,

Chicago, 111., a corporation of Delaware Filed Jan. 2, 1969, Ser. No. 789,112 Int. Cl. F25b 41/04 US. Cl. 62217 3 Claims ABSTRACT OF THE DISCLOSURE Apparatus for controlling the operation of a centrifugal compressor by deriving a signal from the high side of the system and using such signal to reset the control point which limits closing of the PRV and initiates bypassing discharge gas. At lower capacity, the system operates close to the surge line assuring maximum economy.

BACKGROUND AND SUMMARY OF THE INVENTION This invention relates generally to centrifugal compressors and, more particularly, to apparatus for controlling the operation thereof While avoiding surging conditions and maintaining maximum efficiency. Compression in a centrifugal compressor is a result of a rotating impeller acting on a stream of gas and the conversion of the kinetic energy of the gas leaving the impeller into static pressure in the difiusion section. In general, a centrifugal compressor is a constant head, variable capacity machine, and is capable of operating at any point which balances the combined head and flow requirements. For any particular compressor design, an operating curve can be generated by plotting head against flow. In order to increase the operating range in a constant speed drive system, it is common to provide prerotation vanes (PRV) at the suction side or inlet of the compressor. The PRV perform two functions: (1) throttling of the suction gas, and (2) imparting a swirl to the gas in the direction of impeller rotation, as the PRV (vanes) are closed, thereby reducing the compressor capacity.

Under certain conditions, a phenomenon, commonly referred to as surging, may occur in the compressor for example, at a certain head pressure, when the gas velocity through the compressor becomes too low, or at some given gas velocity, when the head pressure becomes too high. Surging may be described as a temporary and intermittent reversal of the gas flow through the impeller resulting from transient pressure imbalance between the suction and discharge. Surging causes excessive vibration and noise, which at the least is annoying, and at worst can cause damage to the compressor.

Various systems to prevent surging have been suggested in the prior art, the most common of which are to bypass gas from the discharge side of the suction side or simply pass it through a blow-off line when the possibility of surging arises. Since the by-passing or blowing-off of gas represents a loss, both in power and efficiency, it is desirable not to initiate this action until the compressor approaches the operating point when a surging condition is presumed to exist.

In the present invention, a control signal is derived which is a function of the pressure on the high side of the system; and, in a conventional refrigeration system, this pressure could be sensed at the condenser. This signal is applied to a pneumatic relay which also receives a signal which is a function of the vane position. Since the vane position gives a reasonably accurate measure of gas flow rate, the combined signal from the pneumatic relay has an output which is a function of both high side pressure and gas flow. This combined signal is then applied to a bypass valve control which will begin by-passing when it interprets the signal as indicating an impending surge condition.

It is therefore a principal object of this invention to provide a novel method of preventing surging in a centrifugal compressor.

Another object of the invention is to provide an improved control system to prevent surging in a centrifugal refrigeration system.

Another object of the invention is to provide a control system which will automatically maintain the minimum PRV opening compatible with the refrigeration load and system head pressure so as to achieve significant operating economies and still assure stable compressor operation.

Still another object of the invention is to provide a low cost, pneumatic control system which is operative to sense an impending surge condition, and will begin by-passing of discharge gas to prevent the same in response to a demand for less capacity.

Additional objects and advantages Will become apparent from reading the detailed description which follows, in conjunction with the drawings.

THE DRAWINGS FIG. 1 is a compressor performance diagram which illustrates the operating characteristics of a typical centrifugal compressor; and

FIG. 2 is a schematic diagram of the control circuit of the present invention.

DETAILED DESCRIPTION OF THE INVENTION In the simplest type of hot gas by-pass system, the capacity control prevents closing of the PRV beyond some predetermined position and then initiates the hot gas bypassing for additional reduction in capacity. This presents somewhat of a problem in that the PRV setting has to be set for the worst condition, i.e., maximum head. Under a low head condition, the vanes could be closed further in response to a demand for reduced capacity without danger of surging.

This may best be illustrated with reference to FIG. 1. This drawing represents the operating curves at different head and flow conditions for various PRV settings. It should be understood that every compressor design has a different set of performance curves; but, for purposes of explaining the present invention, it is only necessary to give an example of a representative compressor.

The curves in FIG. 1 are the locus of operating points (gas flow vs. head) for PRV settings of (1) full open, (2) open, (3) open, and (4) /3 open, respectively. Operating at any point above and/or to the left of the Surge line will result in compressor surging. Just to the right of the Surge line, there is drawn an imaginary Anti-Surge line which, for control purposes, approximates a straight-line locus of operating points in the safe region.

Assume that the system is operating at point A under high head and at maximum capacity with the PRV fully open. Upon a signal for decreased capacity, the PRV will begin to close so that the operating points follow the high head system demand line to point B which lies at the intersection of the demand line and the Anti-Surge line. This point (about /8 open) is where the PRV would be prevented from closing any further (without some reduction in head). An attempt to reduce the flow by closing the PRV beyond this point would cause the compressor to surge. Additional capacity reduction must now be achieved by using hot gas "by-passing. As hot gas is bypassed, operations will follow curve B-C to point C, assuming that the PRV is held at the /8 open position.

In the design of a capacity control not having an Anti-Surge feature, the initiation of hot gas by-passing must always be set for the worst condition. In the case discussed above, this would be the high head condition which would automatically stop the PRV from closing beyond the /8 open position.

During low head conditions, assume that the PRV is full open, and the system is operating at a flow head condition corresponding to point A. If the system signals for reduced capacity, the PRV will begin to close so that operations will follow the low head line (FIG. 1) until it reaches point D. Since the vanes are set with a limit or stop at the 5/8 open position, operations will follow line BDC to point C, just as the system followed this curve after it reached point B when operating at a high head condition. It will be noted, however, that if the limit on the PRV could be re-set to compensate for the change in head conditions, operations could follow the low head operating line to point E on the Anti-Surge line. Point E corresponds to a vane position of about 3/ 8 open. Operations would then follow line E-F to point F; and, this is much more economical to operate the system than point C. Additional control refinements may permit the system to follow the Anti-Surge line very closely throughout the entire range of operations. As the hot gas valve begins to open, a decrease in head is experienced, and the vanes are permitted to open, and the hot gas closes so that the system will operate along curve ABG with the Anti-Surge control system.

Referring now to FIG. 2, there is shown a pneumatic control circuit for automatically re-setting the hot gas by-pass control point in response to a change in head. The compressor C is shown diagrammatically as being supplied with suction gas through line 70 containing PRV or pre-rotation vanes and also includes a by-pass line 72 and a hot gas valve V. The by-pass line extends from up stream of the PRV to the hot gas or discharge line 74. A pressure controller 20 receives a signal through channel 21 which indicates the pressure (or temperature) existing on the high side of the system. This may be a direct acting pressure signal, or generated by a temperature-topressure transducer which, for example, will transmit a p.s.i.g. signal at a high head condition, and a 3 p.s.i.g. signal at a low head condition. It is desirable to have the controller adjustable so that the entire 12 p.s.i.g. range may be utilized.

Control air pressure from some regulated source is supplied to the controller through lines 22, 23, and 24. This same control pressure is applied to other components in the system through lines 25, 26, 27, 28, and 29 as will be described in more detail below.

A pressure regulating valve 30, supplied with control air through line 25, controls the absolute minimum position of the PRV. This corresponds to point G on FIG. 1. During a low head condition, this signal will prevent the vanes from closing past the point where the compressor will surge.

The output signal from controller 20 is applied through line 31 to a second pressure regulating valve 32. This valve controls the minimum vane position during a high head condition (this corresponds to point B on FIG. 1), so the minimum PRV position will always vary between the setting of pressure regulating valves and 32.

The pressure selector valve 34 receives signals from regulating valves 30 and 32 and is operative to always pass the higher of the two input signals. In the embodiment shown, this means that the selector valve will pass the pressure signal on the output side of regulating valve 30 during a low head condition, and will pass on the output signal from regulating valve 32 during a high head condition.

The capacity of the system is controlled in response to suction pressure or leaving chilled water temperature, as is conventional in the art. A control signal is applied through channel 36 into a pressure controller 38. The

4 i control may be a direct acting pressure control responsive to compressor suction pressure, or may be operated by a temperature-to-pressure transducer sensing chilled water temperature. The signal on the output side of pressure controller 38 is applied to an auto-manual control unit 42 which may be used to select either automatic or manual control of the system. If manual control is desired, an adjustable pressure from manual control 44, supplied with control air through line 28, is directed through the auto-manual control. I

The output from the auto-manual control is directed through lines 46, 50, and 48, to a balancing relay 52, and a maximum pressure selector valve 54, respectively. Pressure selector valve 54 will always pass the greater of any two pressures applied to the input side. In this application, it will either select the minimum vane position signal being applied from the output side of pressure selector valve 34 (through line 55), or the capacity control signal, from capacity controller 38, applied through line 50. The output from the pressure selector valve 54 is directed through line 57 and used to control the vane position on the PRV.

The capacity control signal directed through line 48 to balancing relay 52 is used to maintain a predetermined characteristic which is a function of the difference between the pressure signal generated by controller 38 and the pressure output of pressure selector 34. The balancing relay is a fully adjustable relay which has the following characteristics: when the input X (the pressure on the output side of selector 34) is equal to the pressure Y (the pressure on the output side of capacity controller 38), the output Z directed through line 60 to the hot gas valve will be constant, for example, 9 p.s.ig. This is the internal spring setting of the relay, and corresponds to the point where the hot gas valve begins to open. If the input Y falls, the output Z will begin to fall by a corresponding amount. Accordingly, the relay output will be governed by the formula Z:9+(YX). It should be understood that in every case where numerical 'examples are used to simplify the explanation, they are only meant to be illustrative, and not in any sense limiting.

OPERATION OF SYSTEM It will be assumed that the system is operating with the condensing pressure in the high head condition, and that the signal from pressure controller 20 will be at its maximum. The system will, therefore, be operating at point A in FIG. 1. Other conditions of the system are as follows:

(1) Assume that pressure regulator 30 is set at approximately 4 p.s.i.g. (this regulator governs the minimum vane position during a low head condition); and, further, assume pressure regulator 32 is set at 12 p.s.i.g.

(2) Maximum pressure selector 34 will be passing a 12 p.s.i.g. signal to point X on FIG. 2.

(3) Since the capacity is satisfied the output signal from pressure or temperature controller 38 is 15 p.s.i.g., the pressure at point Y is 15 p.s.i.g.

(4) Since the pressure at Y is greater than the pressure at X, the output Z (the signal to the hot gas valve) is greater than 9 p.s.i.g. and the hot gas valve is in the closed position.

(5) Since the pressure at Y is greater than X, the output of maximum pressure selector 54 will be pressure Y or 15 p.s.i.g., and the PRV will be fully open provided that there is no overload on the motor.

As the refrigeration load falls off the system, capacity will be reduced as follows:

The signal from controller 38 (pressure Y) begins to decrease. Since pressure X is approximately 12 p.s.i.g., maximum pressure selector 34 will pass signal Y which begins to close the PRV. As this happens, the operating point on FIG. 1 begins to move from point A to point B. As the reduction in signal reaches approximately 12 p.s.i.g., the operating point should coincide with point B. Any further attempt to reduce the flow would cause the compressor to surge.

Additional reduction in capacity must be accomplished by the addition of hot gas. When signal X is equal to signal Y, the output to the hot gas valve, Z, is 9 p.s.i.g., and the hot gas valve is just beginning to open. For each 1 p.s.i.g. reduction in signal Y, there is a 1 p.s.i.g. output reduction at Z. As the hot gas is added to the system, a decrease in head is experienced which in turn will allow the PRV to close. In this way, the system capacity reduction follows the Anti-Surge line 3-6 on FIG. 1.

The pressure at X is 12 p.s.i.g., and the pressure at Y is 12 p.s.i.g. Any further reduction in signal Y will not have any effect on the PRV, since maximum pressure selector 54 will pass the maximum pressure. This will be pressure X (or 12 p.s.i.g.). Thus, the PRV have reached their minimum position. The system should now be operating at point B.

If, during the course of operation, the condensing head should drop as shown in FIG. 1 (this would indicate a low head condition). The low head line would indicate the new system demand curve.

Using a fixed minimum vane position, capacity would normally be reduced from point A to point D by closing the PRV. At this point, we would add hot gas to the system until we reach point C. Since the flow could have been further reduced by closing the vanes from D to E before addition of hot gas, there is a great deal of inefiiciency in the system.

By re-setting the minimum vane position and not allowing the hot gas valve to open until precisely the time that we reach our minimum vane position, optimum efficiency can be obtained. This is accomplished in the following manner:

As the condensing head decreases, the air signal to pressure regulator 32 will also decrease. This decrease in signal will be passed through maximum pressure selector 34 which will make pressure X somewhat less than at high head conditions. Since pressure X is now less than it was originally, the output of maximum pressure selector 54 will also be less. This will allow the PRV to close further. The PRV reach their minimum position only when pressure X is equal to pressure Y. It should also be noted that output Z to the hot gas valve is 9 p.s.i.g. when pressure X is equal to pressure Y. This always insures that the hot gas valve will begin to open at exactly the time the PRV reach their minimum position.

If the system is at minimum capacity (high head condition), and a decrease in head occurs, the control system will automatically close the hot gas valve and open the PRV to obtain the maximum efficiency.

By way of summary, it can be seen that the capacity control signal, sensing leaving chilled water temperature or some other indication of the demand for cooling, which has passed channel 36 into pressure controller 38 may be regarded as a means for generating a control signal which is a function of the demand. This control signal applied through pressure selector valve 54 is directed through line 57 to the PRV, which operates as a direct capacity controller for the compressor. A second control signal generated by pressure controller is a function of the pressure on the high side of the system, normally taken at the condenser. This second signal is applied through pressure selector valve 34 to one input of pressure selector valve 54, and is operative to override the control signal being passed from pressure controller 38 when the operation would be about to enter a surge condition. At the same time, the balancing relay 52 receives a signal from capacity controller 38 and pressure selector valve 34. With a predetermined constant force built into the balancing relay, the hot gas valve will begin opening at the same time the PRV limit is reached. Since the second control signal is a function of condenser pressure, it is equivalent to a floating limit preventing surging conditions, but still allowing the PRV to close to the lowest possible position before instituting the lay-passing of hot gas.

While this invention has been described in connection with a certain specific embodiment thereof, it is to be understood that this is by way of illustration and not by way of limitation; and the scope of the appended claims should be construed as broadly as the prior art will permit.

What is claimed is:

1. An anti-surge control system for a centrifugal compressor in a refrigeration system of the type which includes first capacity control means operative to regulate gas flow to the suction side of said compressor, said control system comprising means for generating a first control signal which is a function of the demand for cooling on the load side of said refrigerator system; mean-s for applying said first control signal to said first capacity control to regulate the gas flow to said compressor; means for generating a second control signal which is a function of discharge pressure; second capacity control means comprising means defining a gas by-pass path from discharge to suction, and valve means for regulating the flow of by-passed gas; means receiving said first and second control signals and converting said signals to a third control signal said third control signal being a function of the difference in magnitude between said first and second control signals; and means to apply said third control signal to said valve means; and means for overriding said first control signal by said second control signal to establish a floating limit to prevent said capacity control from reducing the gas flow to a surge condition.

2. Apparatus as defined in claim 1, wherein said first capacity control means comprises a pre-rotation vane mechanism.

3. Apparatus as defined in claim 1, including means for generating a low limit signal adapted to override said second control signal when said discharge pressure reaches a predetermined lower value.

References Cited UNITED STATES PATENTS 2,888,809 6/1959 'Rachfal 62196 2,921,446 1/ 1960 Zulinke 62-117 2,983,111 5/1961 Miner 62-228 3,204,423 9/ 1965 Resh 62-217 3,248,896 5/ 1966 Plaster 62-201 MEYER PE RLIN, Primary Examiner US. 01. XJR. 62-228 

